1. Field of the Invention
The invention relates to the fabrication of welded constructions, and more specifically is concerned with an apparatus for limiting propagation of cracks in welded sheet constructions.
The invention can be used to prevent propagation of brittle and ductile fractures in such welded sheet constructions as ship hulls, walls of oil tanks, etc.
The invention is of particular advantage in the case of gas pipelines used at low ambient and operating temperatures, wherein it can be advantageously applied to provide crack arresting means at the places of the most probable paths of avalanche-type propagation of fractures.
2. The Prior Art
There is known a crack arrestor made in the form of guard plates riveted over a through slot provided in a ship hull or in a wall of a reservoir and located in the path of the most probable path of fractures perpendicular to the direction thereof (V. M. Finkel, "Fizika razrusheniya", "Metallurgiya", M., 1970, p. 280). In some constructions, however, riveted joints cannot be used, and in particular in the case of high-pressure gas transmission pipelines wherein riveted joints do not provide the strength and tightness of the pipeline. The use of weld joints in this case also does not yield the required results since a crack, in one occurs, propagates through the weld joints and guard plates further along the pipeline.
There is also known a crack arrestor ("Razrushenie", t.5, Mashinostroenie, M., pp. 40-43) constructed in the form of a stack of plates built-in in the most probable path of crack propagation, which crack arrestor is, in other words, a multi-layer section integrally interposed at a certain place of a construction made of sheets, that is in the place of probable formation of cracks in ship hulls, walls of tanks or at the sections of gas pipelines, fractures in which sections may cause breakdown at other associated installations.
In the case of using a crack arrestor, when a propagating crack reaches the multi-layer stack of plates it changes it velocity and direction, slows down and stops propagating, which arresting effect is produced due to different properties of materials and specific conditions of the propagation of cracks in a multi-layer stack of plates.
The length of such a multi-layer crack arrestor is determined by the length of the crack path, within the crack arrestor, at which said crack can be arrested. In the case of avalanche-type fracturing in a highly pressurized construction, in particular such as a gas transmission pipeline fabricated from large diameter pipes (1200-1400 mm) the velocity of crack propagation may be as high as 400-600 m/s. It is therefore quite evident that such a crack arrestor should have a large size in the direction of probable fracture path, and hence, correspondingly expensive. In addition, in the case of ductile fracturing the fracture limiting effect will be of a low efficiency since the ductile properties of the said stack of plates having the same thickness will be close to the corresponding properties of the material in which ductile fractures occur.